stanbnv

I drove truck in the late 60's and at the time I was told that RPM should be held between 1800 - 2100, (220, & 235 Cummins and 318 Gimmy). My '06 3500 CTD/w 6sp manual supposedly has a flat torque curve between 1600 and 2800 RPM and the red line on the tach is 3200. I may rev to 2500 when pulling out on the highway, and in 6th gear at 65 mph my tach reads about 2250, and at 55 mph 1750. I normally tow my 10,000 lb 5th wheel at about 60 mph at about 2100 RPM. Since I don't have any more gears to shift up to, what would be the optimum RPM for this engine?

cdennyb

peak torque will usually make the best mpg and pulling ability. I pull our 40' 3-axle at 60 on occassion but the mpg suffer after that. 58 is about 1800 and the motor really likes it there.

stanbnv

The problem with peak torque is that, if I remember right, it is around 2800 RPM. I think that is a little too fast.

SundayMoney

2100 is the sweet spot while towing my 11,000lb fifth wheel. At those rpm it'll pull just about any highway grade without losing speed. It doesn't pull nearly as well at less than 2000.

CoastalDav

Cummins says what you have in gearing is correct for best fuel mileage and power while towing. I also tow at 65, but I can't get were your at unless I either put 4.56 gears in or smaller tires on the back. They don't make a 4.30 rear either.

http://www.powerspec.cummins.com/sit...ng/gearing.htm

At 2250 the moving mass makes those hills easy to do. Click on ISB at the link and select. It will tell you want is the best rpm to set the truck up for. Your there at 2100 to 2250.

Dave

Seabiscuit

Hey that was a a pretty cool tool from Cummins. Interesting, It seems to feel better between 1800 & 2100. I always thought it was a little 'buzzy' over 2100. Around 1950 rpm gets me close to 64 mph, when towing, I'm good with that.
When not towing our travel trailer, it always feels like there is so much power. Like it just overpowers the truck, You know what I mean? Even light on the accelerator. I wish sometimes the 48re had a fifth gear sometimes.
It will pull our 21 foot Outback, all day long and not break a 'sweat'.